1. Field of the Invention
The present invention relates to hydraulic accumulators and more particularly, relates to a compact hydraulic accumulator suitable for use with a power pack of an automatic door actuator of the type as shown in U.S. Pat. No. 3,620,014 which patent is assigned to the same Assignee as the present application.
With the increasing usage of electrically controlled automatic doors for entrances of commercial establishments and the like, it has been a problem to provide a door actuator which is powerful enough to open a relatively heavy door in rapid fashion and then close the door after the traffic has passed through the entrance. This problem is most readily solved by use of an accumulator to store hydraulic fluid under pressure for release as required and particularly there is a need for an accumulator which is reliable and operates with a minimum of service and maintenance difficulties. Because of the relatively high power requirements in these types of applications, it has been necessary to utilize hydraulic systems which employ relatively high pressure and thereby reduce the size of the door operator unit so that it can be hidden from view by mounting in a transom bar or the like above the door entrance. This arrangement results in greater architectural flexibility in the design of entrances and does not require a large, unsightly mechanism as often are common with other types of door operators. In addition, in establishments such as super markets and the like, it is neccessary to provide a rapid door opening cycle to accommodate the large volume of traffic flow and this in turn requires a relatively high power system which is capable of developing high energy during short periods of time to handle the large heavy duty type entrance doors.
2. Description of the Prior Art
In the aforementioned U.S. Pat. No. 3,620,014, therein is illustrated an automatic door actuator employing a power pack which is electrically actuated by an electric control module in turn actuated by the presence of traffic on a switch mat or the like. The control module may also be activated by other traffic presence detectors and when activated to open a door, the control module activates a control valve in the power pack in order to supply high pressure hydraulic fluid to a hydraulic door actuator which rapidly swings the door to an open position. As the actuator opens the door, a return spring is compressed and once the traffic is clear of the entrance, the control spring then returns the door back to a closed position. The hydraulic actuator of such a unit requires a relatively high rate of flow of high pressure fluid to cause the door to swing open rapidly, that is with enough speed to accommodate the high volume traffic rates required. Accordingly, the motor powered hydraulic pump of the power pack must be supplemented with additional hydraulic fluid under pressure available from an accumulator or pressurized reservoir.
Accumulators and reservoirs of this type are commonly pressurized by means of a charge of nitrogen gas acting on one side of a flexible diaphragm or piston to pressurize the hydraulic fluid in a chamber on the adjacent opposite side. Because in theory, the pressure of the nitrogen gas and hydraulic fluid is equal, there are normally few leakage problems during operation of these types of accumulators. However, when an accumulator is shut down, for instance at night time, the nitrogen gas tends to leak past the seals on the pistons and the like and in addition, because the nitrogen charge is usually lost if servicing of the unit is required, servicing is difficult and costly and unskilled maintenance or servicemen cannot be used. In addition, because accumulators of the nitrogen gas type are often located in positions where the temperatures may vary in a wide range, the sealing and gas leakage problems often are acute and greatly amplified and many times when sealing failures occur, it is necessary to return the whole pack unit to the factory for a complete overhaul or rebuilding job by skilled personnel to insure continued operation. In the particular entrance door application as shown in the aforementioned U.S. Patent, an electric switch for controlling the pump motor of the system is positioned in the nitrogen filled chamber, and replacement of the switch is often difficult, In addition, sealing around the electrical leads, where the leads pass out of the gas chamber is another area where leakage of nitrogen may occur.